In ZnO, The PL shifted by changing the excitation wavelength. Read the attached articles.

Conference Paper Observation of excitation wavelength dependent photoluminesc...

The PL emission peak positions of the phosphorene quantum dots are not red-shifted with progressively longer excitation wavelengths, which is in contrast to the cases of graphene and molybdenum disulphide quantum dots.

https://www.nature.com/articles/srep27307

typically not.

The idea is that independently from the excitation w.l. the emission mediated relaxation happen between the same energy levels.

this is of course a very plain explanation in a very simple system.

Now you can have a lot of different reason why the Pl position does depend on the excitation w.l. , excitation of different sub-population, charge transfer intra-system, scattering of any material in the photon path.

I think you´ll have to be a bit more specific on your system and observed effect

Just a follow-up comment to Bruno Pescara's answer.

You may want to tune the excitation wavelength to be resonant or non-resonant to certain transition levels. For example, you want to suppress the luminescence caused by free excition recombination. Hence, you would like to go to sub-band gap energies for the laser. So, you might get a better understanding about the luminescence related to defect levels.

Usually that doesn't happen. The whole concept of the quantum dot is that the emission w.l. depends on the size of the nanoparticle and not on the excitation w.l. Maybe in your systems occur one or many different supramolecular interactions ,likely due to the functional groups attached to the surface of your QD, that cause that unusual behavior.

First of all thank you for all the replies, much appreciated.

I am following a modified safe and green organometallic synthesis method witch includes 2 precursor - 1) CdO powder, oleic acid, olive oil 2) Se powder, olive oil. Both solution heated to a desired temperature and added together, temperature maintained for nucleation and growth of QDs, samples extracted at different timing and added into acetone, so that example red shift of emission wavelength can be studied.

However at my level of research, in-depth measurement and understanding of what you guys stated is very difficult.

Of course not. Property is determined by the structure (dimensional effect) of QDs.From the point of view of synthetics, you used the" dirty" method of synthesis of CdSe. It includes heating, decomposition of organic substances that are difficult to purify by dissolving in acetone. Better to get quantum dots by exchange reaction

Cd2+ + Se2- =CdSe

Get other nanoparticles and you will get other indicators of luminescence in the device.